Tracker of a surveying apparatus for tracking a target

ABSTRACT

The present invention relates to a tracker and a surveying apparatus comprising the tracker, which improve the reliability of tracking a target. The tracker comprises a first imaging region having a plurality of pixels for taking a first image of a scene including the target; a second imaging region having a plurality of pixels for taking a second image of a scene including the target; a control unit to receive a timing signal indicating a time duration during which an illumination illuminating the target in the scene is switched on and off, control the first imaging region to take the first image of the scene when the timing signal indicates that the illumination unit is switched on, and control the second imaging region to take the second image when the illumination is switched off; and a read out unit configured to read out the first image from the first imaging region and the second image from the second imaging region and to obtain a difference image.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.16/567,723, filed Sep. 11, 2019, which claims priority to EPCApplication No. 18200908.4, filed Oct. 17, 2018, the contents of whichare incorporated herein by reference in their entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to a tracker and a surveying apparatuscomprising the tracker as well as a method for tracking a target, and inparticular, to a tracker having at least one imaging region with aplurality of pixels on an image sensor arrangement.

BACKGROUND

Optical instruments, such as geodetic or surveying instruments orapparatuses are commonly used for measuring a position of an object toobtain information, such as horizontal and vertical angles and distance.Newer instruments are often provided with a tracker, a.k.a. trackingunit or system, and an electronic imaging device, e.g. a camera, toacquire digital images of the object.

A conventional surveying instrument comprises a telescope system forsighting an object which can then be imaged on a camera behind thetelescope system. Further, such an instrument may comprise a distancemeasurement unit to measure a distance to the object sighted by thetelescope system. The viewing angle of the telescope system is generallyvery small, e.g. 1 to 2 degrees, and a user has to position thesurveying instrument and adjust the optics of the telescope system sothat the object to be sighted and to be measured is exactly in the smallfield of view of the telescope system and optimally on the optical axisof the telescope system, e.g. to measure a distance to the object.

However, in certain cases the object may move so that the user has toreadjust the instrument each time anew before sighting the object and/ormeasuring the distance to the object.

Recently, surveying instruments with tracking systems have been proposedto automatically follow the position of a moving target, i.e. an object.For example, a laser tracker comprising a laser beam may be used totrack an object. Thereby, a pivotable mirror may be used to deflect thefocused laser beam in the direction of the object and the direction maythen be recorded using the angles of the mirror position, for examplefor changing the optical axis of the instrument for distancemeasurement.

However, tracking a target is difficult and usually requires that afocused laser beam or other directed radiation hits and is reflectedfrom a reflector at the target back to the tracker. It can be hard tomeet this condition, especially when the target is moving quickly and/orwhen operating in a bright environment with a lot of background lightand/or additional other reflections from the sun or other light sources.

It is also possible that the reflector (reflective target) when movingbecomes temporarily occluded by an obstacle. This would also rendertracking temporarily impossible leading to a target loss, or the trackermay get confused and erroneously consider reflections by the sun orother light sources as reflections of the target reflector.

SUMMARY OF THE INVENTION

Therefore, there is a need to improve the reliability of tracking atarget and/or to decrease the search time needed by a tracker whensearching for a target.

According to an embodiment, a tracker for tracking a target comprises afirst imaging region having a plurality of pixels on an image sensorarrangement for taking a first image of a scene including the target anda second imaging region having a plurality of pixels on the image sensorarrangement for taking a second image of a scene including the target.The tracker further comprises a control unit configured to receive atiming signal, wherein the timing signal indicates a time durationduring which an illumination unit illuminating the target in the sceneis switched on and off. The control unit controls the first imagingregion to take the first image of the scene when the timing signalindicates that the illumination unit is switched on, and controls thesecond imaging region to take the second image when the timing signalindicates that the illumination unit is switched off. The trackerfurther comprises a read out unit to read out, e.g. after taking thefirst and the second image, the first image from the first imagingregion and the second image from the second imaging region and to obtaina difference image by determining a difference between the pixel valuesof the pixels of the first imaging region and the second imaging regionso as to identify the target in the difference image. Accordingly, thesensitivity in detecting and tracking a target is increased, since, forexample, bright or reflecting surfaces which could be confused with atarget can be largely eliminated in the difference image. In particular,using two imaging regions allows the time between taking a first imageand second image to be set short. As a result, tracking can be performedmore reliably decreasing target losses when tracking a target anddecreasing the time in finding a target. Further, the distance to thetarget can be increased, since the reduced noise allows for longerexposure times.

According to an advantageous embodiment, the image sensor arrangement ofthe tracker comprises a first and a second tracker receiver includingthe first and second imaging region, respectively, wherein each trackerreceiver receives part of back-reflected tracking light split by a beamsplitter. Accordingly, simple off-the-shelve camera chips can be used astracker receivers which independently record a first and second image ofbasically the same scene, respectively.

According to an advantageous embodiment, the tracker is calibrated bysetting the center of the second imaging region depending on the opticalcenter of the first imaging region. Accordingly, alignment of the twoimaging regions with respect to each other is simplified, since oneimaging region can be set based on the other imaging region. Forexample, the first imaging region is glued in the tracker first and thesecond imaging region can be arranged making use of six degrees offreedom to obtain good alignment. Accordingly, a precise and simplealignment of the imaging regions can be performed.

According to an advantageous embodiment, the read out unit corrects,after taking the first and the second image, for a movement of thetracker in the time between taking the first and the second image bytaking into account an offset between the scene on the first image andthe scene on the second image, the offset corresponding to a shift inthe scene due to the movement of the tracker. Accordingly, evenmovements of the tracker between taking the first image and the secondimage can be electronically corrected to improve the reliability of thetarget detection and avoid wasting time by taking images in a“stop-and-go” mode.

According to an embodiment, a tracker for tracking a target comprises animaging region having a plurality of pixels on an image sensorarrangement for taking a first image of a scene including the target,for each pixel charges being collected in a charge storage correspondingto the image information of the first image, and for taking a secondimage of the scene including the target, for each pixel a charge valuecorresponding to the image information of the second image being removedfrom the charge value of the collected charges corresponding to thefirst image so as to generate a difference image. The tracker furthercomprises a control unit receiving a timing signal which indicates atime duration during which an illumination unit illuminating the targetin the scene is switched on and off. The control unit controls theimaging region to take the first image of the scene when the timingsignal indicates that the illumination unit is switched on, and controlsthe imaging region to take the second image when the timing signalindicates that the illumination unit is switched off. The trackerfurther comprises a read out unit to read out, after taking the firstand the second image, the difference image from the image sensorarrangement so as to identify the target in the difference image.Accordingly, the sensitivity in detecting and tracking a target isincreased, since, for example, bright or reflecting surfaces which couldbe confused with a target can be largely eliminated in the differenceimage. In particular, generating a difference image directly on an imagesensor arrangement before a time consuming read out of each pixel allowsthe time between taking a first image and second image to be set short.As a result, tracking can be performed more reliably decreasing targetlosses when tracking a target and decreasing the time in finding atarget.

According to an advantageous embodiment, the control unit is configuredto reverse, after taking the first image, the polarity of pixels, so asto remove, in the sense of subtract or neutralize, the chargescorresponding to the image information of the second image from thecharges collected in the charge storage corresponding to the imageinformation of the first image. Accordingly, a simple electrical circuitallows for generating a difference image directly in an image sensorarrangement.

According to an advantageous embodiment, the tracker comprises a trackeremitter for emitting tracking light on an optical tracker path. Thetracker emitter may include the illumination unit so that the trackinglight illuminates the target. Accordingly, tracking light may beprovided which can be reflected by a reflective target in the scene soas to easily detect the target.

According to an advantageous embodiment, the tracker is adapted to issuean instruction to a surveying apparatus to move the optical axis of alens arrangement of the surveying apparatus. Accordingly, the trackercan be used to autonomously control the movement of a lens arrangementso that a surveying apparatus can automatically follow a target and takeimages during its movement and/or measure the distance to the target.

According to an embodiment, a surveying apparatus is provided comprisinga lens arrangement including at least one movably arranged focus lenselement for focusing to sight a target; an imaging unit configured toobtain an image of at least a part of the target; and the trackerdescribed above. Accordingly, a surveying apparatus may benefit from theadvantages of the above described tracker.

According to an advantageous embodiment, the surveying apparatus furthercomprises a beam splitter/combiner configured to combine a part of theoptical imaging path of the imaging unit and a part of the opticaltracker path of the tracker so that the optical axis of the imaging unitand the optical axis of the tracker are at least coaxially arranged withthe optical axis of the lens arrangement between the lens arrangementand the beam splitter/combiner. Accordingly, a compact optical setup isachieved in which the optical paths of the tracker and the imaging unitare combined so that the same lens arrangement is used for both.

According to an advantageous embodiment, the surveying apparatus furthercomprises a distance measuring unit configured to measure a distance tothe target along the optical axis of the distance measuring unit.Accordingly, a distance to a target can be measured while the target istracked.

According to an advantageous embodiment, the beam splitter/combiner isfurther configured to combine the part of the optical tracker path ofthe tracker, the part of the optical imaging path of the imaging unitand a part of the optical distance measuring path of the distancemeasuring unit so that the optical axis of the tracker, the optical axisof the imaging unit and the optical axis of the distance measuring unitare at least coaxially arranged with the optical axis of the lensarrangement between the lens arrangement and the beam splitter/combiner.Accordingly, a compact optical setup is achieved in which the opticalpaths of the distance measuring unit, tracker and the imaging unit arecombined so that the same lens arrangement is used.

According to an embodiment, a method for tracking a target comprisestaking a first image of a scene including the target on a first imagingregion having a plurality of pixels; and taking a second image of ascene including the target on a second imaging region having a pluralityof pixels. A timing signal is received before taking the images, thetiming signal indicating a time duration during which an illumination ofthe target in the scene is switched on and off, and the first imagingregion is controlled to take the first image of the scene when thetiming signal indicates that the illumination is switched on and thesecond imaging region is controlled to take the second image when thetiming signal indicates that the illumination unit is switched off. Themethod further comprises reading out the first image from the firstimaging region and the second image from the second imaging region andobtaining a difference image by determining a difference between thepixel values of the pixels of the first imaging region and the secondimaging region so as to identify the target in the difference image.Accordingly, the same advantages as mentioned above can be achieved; inparticular, tracking can be performed more reliably decreasing targetlosses when tracking a target and decreasing the time in finding atarget.

According to an embodiment, a method for tracking a target comprisestaking a first image of a scene including the target on an imagingregion having a plurality of pixels, for each pixel charges beingcollected in a charge storage corresponding to the image information ofthe first image; and taking a second image of the scene including thetarget on the imaging region, for each pixel a charge valuecorresponding to the image information of the second image being removedfrom the charge value of the collected charges corresponding to thefirst image so as to generate a difference image. A timing signal isreceived before taking the images, the timing signal indicating a timeduration during which an illumination of the target in the scene isswitched on and off; and the imaging region is controlled to take thefirst image of the scene when the timing signal indicates that theillumination unit is switched on, and the imaging region is controlledto take the second image when the timing signal indicates that theillumination unit is switched off. The method further comprises readingout the difference image from the imaging region so as to identify thetarget in the difference image. Accordingly, the same advantages asmentioned above can be achieved; in particular, tracking can beperformed more reliably decreasing target losses when tracking a targetand decreasing the time in finding a target.

According to an embodiment, a program is provided including instructionsadapted to cause data processing means to carry out the above methods.

According to another embodiment, a computer readable medium is provided,in which the program is embodied, where the program is to make acomputer execute the above methods.

According to another embodiment, a surveying system is providedcomprising a remote control unit and the above described surveyingapparatus, wherein the surveying apparatus comprises a communicationinterface to communicate with the remote control unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates elements of a surveying apparatus.

FIG. 1B illustrates elements of a surveying apparatus according to anembodiment.

FIG. 1C illustrates elements of a surveying apparatus according toanother embodiment.

FIG. 2 illustrates a detailed embodiment of a surveying apparatus.

FIG. 3 illustrates a part of a surveying apparatus having an alternativearrangement of functional modules.

FIG. 4 illustrates a specific embodiment of a tracker which can be usedtogether with the surveying apparatus.

FIG. 5 illustrates a specific embodiment of a surveying apparatusincluding a tracker and an optical tracker path.

FIG. 6 illustrates elements of a surveying apparatus in a surveyingsystem according to another embodiment emphasizing the communication andcontrol between elements on a functional level.

FIG. 7A illustrates functional elements of a tracker according to anembodiment.

FIG. 7B illustrates qualitatively the function of a tracker according toan embodiment.

FIG. 8A illustrates schematically an embodiment of a tracker having onlyone tracker receiver including one imaging region providing a differenceimage.

FIG. 8B illustrates qualitatively the function of a tracker according toanother embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention are described with reference tothe Figures. It is noted that the following description containsexamples only and should not be construed as limiting the invention. Inthe following, similar or same reference signs indicate similar or sameelements or functions.

Embodiments of the invention generally relate to trackers having animage sensor arrangement with a plurality of pixels so as to provide oneor two imaging regions. By taking two images a difference image can begenerated using the trackers. In particular, the trackers are designedsuch that for taking a second image of a scene including a target, thereis no need to wait for reading out the image information of a firstimage of the scene taken first, since one tracker comprises twoindependent imaging regions to generate a difference image and the othertracker generates a difference image directly in the image senorarrangement.

A calculated difference image preferably shows only reflectors likeprisms, cat eyes or reflective foils as potential targets whilesuppressing reflections and other light from other sources. To recognizedirection and amount of movements of the target with a high update rateand to avoid artefacts caused by moving sun reflections or other stronglight sources it is important to minimize the time between the exposuremoments of the two images used to calculate the difference image. Thistime minimum is usually defined by the read out time of the camerasensor and takes for a useable sensor like the Wide-VGA sensor MT9V034from Aptina about 16 ms. Using the trackers presented herein, the timebetween two exposures can be reduced far below this time minimum.

In the following, first different optical setups of surveyingapparatuses are discussed, some of which include a tracker, whereindetails of the trackers of different embodiments are discussed in thecontext of the relevant figures.

FIG. 1A illustrates elements of a surveying apparatus 100A. Thesurveying apparatus 100A comprises a lens arrangement 110, an imagingunit 120, a distance measuring unit 140 and a beam splitter/combiner150.

The lens arrangement 110 is provided to sight an object which is atarget, such as a reflector. The optical arrangement 110 includes atleast one movably arranged focus lens element 116 for focusing so as tosight the object. For example, the focus lens element may be anindividual or compound focusing lens which is moved manually orautomatically to produce in focus/out of focus images of the objectwhich may be viewed through an ocular constituting a simple imagingunit. The lens arrangement 110 may be part of a telescope known in theart of surveying, and may comprise several optical lenses, such aslenses 114 and 116, so as to enable focusing and zooming. In oneexample, the lens arrangement 110 is attached to a positioning unit soas to change the direction of the optical axis of the lens arrangementwhich will be discussed in more detail with respect to FIG. 6 .

The imaging unit 120 is configured to obtain an image of at least a partof the object sighted by the lens arrangement 110. The imaging unit 120may be a simple lens or an ocular, a.k.a. eyepiece, so that an image canbe obtained in the eye of the user. Alternatively, the imaging unit 120may be a combination of an electronic imaging device, a micro-displayand an ocular so that an image can be recorded and viewed conventionallyby eye through an ocular. Preferably the imaging unit 120 is anelectronic imaging device, for example, a two-dimensional array ofsensor elements capable of generating image information with a number ofpixels generally corresponding to the number of elements of the array,such as a charge-coupled device (CCD) camera or a complementarymetal-oxide semiconductor (CMOS) camera. Such a sensor array may becomposed of 1000×1000 sensor elements or more to generate digital imageswith 10⁶ image pixels (1 megapixel) or more. However, also smallersensor arrays are feasible, for example, composed of 480×750 sensorelements, for example. Alternatively, the sensor array may be composedof avalanche photodiodes (APD) forming an APD array.

The distance measurement unit 140 is configured to measure a distance tothe object along the optical axis of the distance measuring unit 140,which constitutes an optical measurement axis. For example, the distancemeasurement unit 140 uses an electronic distance measurement (EDM) toobtain measurement values about the distance. In one example, thedistance measurement unit 140 includes a coherent light source, such asan infrared laser or another suitable laser, e.g. emitting in ared-wavelength range, and preferably a fast reflector-less working EDM.Conventionally collimated light is sent out in a radial direction fromthe surveying apparatus 100A to perform a distance measurement by usinga pulse method or phase method as known in the art. Further, theintensity of a received EDM-Signal, i.e. the back-reflected signal of anelectro-optical distance measurement, could also be used to obtaininformation about the distance to the reflecting object. A preferredembodiment of a distance measuring unit 140 will be discussed withrespect to the distance measuring unit 240 in FIG. 2 .

In FIG. 1A, a beam splitter/combiner 150 is provided and configured tocombine a part of the optical imaging path of the imaging unit 120 and apart of the optical distance measuring path of the distance measuringunit 140 so that the optical axis of the imaging unit 120 and theoptical axis of the distance measuring unit are coaxially arranged withthe optical axis of the lens arrangement at least between the lensarrangement 110 and the beam splitter/combiner 150.

An optical axis may be regarded as an imaginary line that defines thepath along which light propagates through the system, up to a firstapproximation. For a system composed of simple lenses and mirrors, anoptical axis passes through the center of curvature of each surface, andcoincides with the axis of rotational symmetry. The optical path may beregarded as the path that light takes when traversing an optical system,such as the lenses of the lens arrangement 110. The optical (beam) pathis usually limited by a three-dimensional volume having the optical axisas an axis of rotational symmetry, in which light may travel. An exampleis provided with respect to FIG. 5 which is discussed below in moredetail.

The optical axis of the imaging unit 120 is shown by the dot-and-dashline and the optical axis of the distance measuring unit 140 is shown bythe dashed line. No preferred direction is given by the lines (opticalreciprocity) but it is understood that light to be imaged on the imagingunit 120 travels from left to right. The beam splitter/combiner 150combines these lines to obtain an overlap of the optical axes on theleft part of the beam splitter/combiner 150 in FIG. 1A. Since these axesare parallel and overlapping, i.e. coaxial, to each other, andadditional parallel and overlapping to the optical axis of the lensarrangement 110, they are also regarded coaxially arranged with respectto the optical axis of the lens arrangement 110.

In particular, the optical setup and especially the beamsplitter/combiner 150 are chosen such that the optical axis of the lensarrangement 110 corresponds to the overlapping optical axes of theimaging unit 120 and the distance measuring unit 140 between the beamsplitter/combiner 150 and the lens arrangement 110 as well as along thelens arrangement 110 so that light traveling in the respective opticalpaths is affected by the lenses 114 and 116 of the lens arrangement.Since the optical axes of the distance measuring unit 140 and theimaging unit 120 partly overlap, also the optical paths, i.e. theoptical imaging path and the optical distance measuring path, of theseunits partly overlap when being combined by the beam splitter/combiner150. In the apparatus, the center of the two dimensional array of sensorelements as imaging unit and the center of the tracking unit do not needto coincide with the optical axis of the lens arrangement 110. Forexample, in a calibration step, the center can be defined on the twodimensional array of sensor elements as the point where the optical axiscoincides with the array.

It is understood that a coaxial arrangement of two or more optical axesis basically a theoretical assumption, since in practice the opticalaxes will usually not exactly overlap and point in the exact samedirection but will overlap within some small error range. Thus, for easeof explanation we assume axes deviations of less than +/−0.2° still ascoaxial. Typical alignment errors are in the order of +/−0.1° which canbe corrected later in calibration by software.

Looking at FIG. 1A from a different perspective, according to theoptical reciprocity principle, light originating on the left side inFIG. 1A, and thus entering the lens arrangement 110 from the left, willbe split by the beam splitter/combiner 150. Therefore, describing thatan optical path is split into two optical paths by the beamsplitter/combiner (looking from left to right) is technically the sameas describing that two optical paths are combined by the beamsplitter/combiner (looking from right to left).

In a simple case, a semi-transparent mirror may be used as beamsplitter/combiner dividing the incoming light into two parts, e.g.50:50, one part reaching the imaging unit 120 and the other partreaching the distance measuring unit 140. Undesired distance measuringlight in the imaging channel may then be filtered before it hits theimaging unit 120. In practice, however, a dichroic mirror or prism, i.e.a mirror or prism which is transparent for one wavelength range andreflective for the other, is used. This wavelength selectivity may beachieved with dichroic filters/films using the principle of thin-filminterference. Using a dichroic mirror or dichroic prism thus allowsusing a large percentage of reflected and transmitted light,respectively.

Accordingly, depending on the light direction and the wavelength, inaddition to its configuration to combine light beams, the beamsplitter/combiner 150 is also configured to split light reflected fromthe object traversing the lens arrangement in imaging light along theoptical imaging path and in distance measuring light along the opticaldistance measuring path.

It is clear from the above that the optical paths and optical axes areindependent of the light traveling direction so that “splitting” and“combining” is merely used to better explain the optical layout. Inparticular, the imaging unit in these examples only receives light anddoes not send out any light so that the beam splitter/combiner does notcombine light from the imaging unit and the distance measuring unit butis configured with an optical function that could do so, since lightentering the surveying apparatus through the lens arrangement is splitin different channels by the beam splitter/combiner. In other words, theoptical function of the beam splitter/combiner is to combine differentpaths from its right side to overlap on its left side.

In one example, a laser diode of the distance measuring unit 140 mayemit light in the red range of approximately 660 nm (or 635 nm) and theimaging unit 120 may image a scenery including an object reflectingvisible wavelengths. Accordingly, if a dichroic mirror with a cut-offwavelength of approximately 620 nm, i.e. wavelengths larger than 620 nmare reflected, is provided (alternatively a notch filter blocking lightaround 635 nm), distance measuring and imaging may be achieved inseparate channels with hardly any loss in intensity. Using a dichroicprism design further allows to glue a camera chip of an imaging unitdirectly onto parts of the prism so that a highly compact structure isachieved which is largely insensitive to temperature changes andexternal shocks while mechanic components for attaching and aligning acamera chip can be saved.

Additional reliability of the measurements of the surveying apparatuscan be achieved if the lens 116 in the lens arrangement 110 facing thebeam splitter/combiner has a convex side, e.g. a plano-convex or abi-convex lens, which faces the beam splitter/combiner. As a result,reflections from this lens of distance measuring light from the distancemeasuring unit 140 may not be reflected back to the distance measuringunit 140 so as to avoid crosstalk which could lead to the detection ofsignals not coming from the actual target (object). Furthermore,anti-reflection coatings on the lenses of the lens arrangement may alsoreduce crosstalk. When using a prism as the beam splitter/combiner 150,an intermediate focus should be placed outside and not inside the prismand the surface(s) of the prism on which light is incident may beslightly tilted with respect to an orthogonal direction so that light isnot fully orthogonally incident thereon. Furthermore, air gaps betweenindividual prisms for total reflection can be provided whereappropriate.

In FIG. 1B, an embodiment of a surveying apparatus is provided whichfurther builds on the surveying apparatus 100A of FIG. 1A. Specifically,the surveying apparatus 100B comprises the same elements as thesurveying apparatus 100A and additionally comprises a tracker 130.

The tracker 130 is configured to track the object, e.g., a triple prismreflector as target, by using preferably infrared light at a wavelengthof 850 nm (or 810 nm) as illumination light. As directly understandablefrom FIG. 1B, the beam splitter/combiner 150 of FIG. 1A needs somemodification to combine/split the three beam paths of the tracker 130,the imaging unit 120 and the distance measuring unit 140, respectively.Thus, the beam splitter/combiner 150′ is configured in FIG. 1B tocombine a part of the optical tracker path of the tracker 130, a part ofthe optical imaging path of the imaging unit 120 and a part of theoptical distance measuring path of the distance measuring unit 140 sothat the optical axis of the tracker, the optical axis of the imagingunit and the optical axis of the distance measuring unit are coaxiallyarranged with the optical axis of the lens arrangement 110 at leastbetween the lens arrangement and the beam splitter/combiner 150′. Thus,the lens arrangement 110 is shared by the tracking, distance measuringand imaging functions.

In more detail, in FIG. 1B, the optical axis of the tracker 130 is shownby the dotted line 131, the optical axis of the imaging unit 120 isshown by a dot-and-dash line 121 and the optical axis of the distancemeasuring unit 140 is shown by the dashed line 141. In FIG. 1B, it isschematically shown how light of these optical axes is reflected andtransmitted by the beam splitter/combiner 150′ to coincide with theoptical axis 111 of the lens arrangement 110.

The prism system shown in FIG. 1B is a multi-channel prism. Inparticular, the prism system comprises two prisms having wedge shapes.In a preferred embodiment, the beam splitter/combiner 150′ comprises atleast two wedge shaped prisms and wavelength selective surfaces. Awavelength selective surface is any surface which reflects differentwavelengths differently. In the above example of the dichroic mirror (orsimilar dichroic prism), the dichroic mirror (or dichroic prism) mayalso comprise a wavelength selective surface. The more optical pathsneed to be combined, the more prisms or mirrors (or combinationsthereof) need to be provided. Thus, in a preferred embodiment havingthree functional modules, such as tracker 130, imaging unit 120 anddistance measuring unit 140, the prism system is made up of two dichroicprisms having dichroic mirror-like surfaces.

The skilled person realizes that instead of using preferably the twodichroic prisms shown in FIG. 1B also two dichroic mirrors may be used.Therefore, similar to FIG. 1A, the optical axes of the tracker, distancemeasuring unit and imaging unit can be coaxially arranged with theoptical axis of the lens arrangement 110 on the left side of the beamsplitter/combiner 150′.

In one embodiment, the tracker 130 comprises a tracker receiver and atracker emitter. For example, the tracker emitter is disposed togetherwith the tracker receiver in the same tracker unit 130. In this example,the tracker emitter emits tracking light on the optical tracker path,i.e. along the optical axis 131 and 111. Details of such an arrangementare provided with respect to FIG. 5 .

In another embodiment, the tracker 130 comprises two tracker receiverseach receiving a part of back-reflected tracking light split by a beamsplitter, e.g. prism cube. In this example, the tracker emitter may beplaced at one end of the lens arrangement 110, for example, and mayconstitute a ring of LEDs around an opening of the lens arrangement.

The tracking light may have a wavelength in the infrared range, such as850 nm, the distance measuring light may have a wavelength in the redrange, such as 633 nm, e.g. from a He—Ne-Laser, and the imaging unit mayreceive visible light below the red distance measuring light. The prismsystem 150′ may then be provided with suitable wavelength selectivesurfaces to guide outgoing light to and through the lens arrangement 110and separate incoming light to reach the three individual channels.

Accordingly, the incoming light which may comprise tracking lightreflected from the object, ambient light reflected from the object aswell as from other structures in the field of view of the surveyingapparatus and distance measuring light reflected from the object, entersthe lens arrangement 110, wherein the beam splitter/combiner 150′ isconfigured to split this reflected light from the object (as well asother structures) traversing the lens arrangement into tracking lightalong the optical tracker path 131, into imaging light along the opticalimaging path 121 and into distance measuring light along the opticaldistance measuring path 141.

In another embodiment, the surveying apparatus 100A, 100B furthercomprises a thermal imaging camera configured to acquire an image of atleast part of the object in a wavelength range above the visible range.For example, the thermal imaging camera may be an infrared camerareplacing the imaging unit 120 in FIG. 1A or 1B, or the tracker 130 inFIG. 1B. The thermal imaging camera may also be provided in addition tothe three functional modules 120, 130, and 140 in FIG. 1B so that adifferent prism system with one more channel needs to be provided.

FIG. 1C illustrates elements of another example of a surveying apparatus100C. The surveying apparatus 100C comprises a lens arrangement 110, animaging unit 120, a tracker 130 and a beam splitter/combiner 150. Thesurveying apparatus 100C corresponds to the surveying apparatus 100A butthe distance measuring unit 140 is replaced by the tracker 130, whereinspecific trackers which can be used as tracker 130 are discussed belowwith respect to FIGS. 7A and 7B as well as 8A and 8B. In the surveyingapparatus 100C the optical axis of a distance measuring unit (notshown), if included, may be parallel to the optical axis of the lensarrangement.

The surveying apparatus 100A, 100B or 100C may be integrated in orconstituted by a video surveying instrument, such as a video theodoliteor a video tacheometer, also known as a tachymeter or total station orany other kind of optical instrument used for surveying, and inparticular for determining angles and distances to an object to derivethe position of the object.

Two tracker designs will be explained in more detail in the following,each of which can be implemented in the surveying apparatuses describedwith respect to FIGS. 1B and 1C as well as FIGS. 2, 3, 5 and 6 .

The first tracker design is depicted in FIG. 7A. This tracker 700comprises a first imaging region 710, a second imaging region 720, acontrol unit 730 and a read out unit 740.

The first imaging region 710 having a plurality of pixels may be a partof a first tracker receiver, e.g. tracker receiver 235 of FIG. 2 , andthe second imaging region 720 having a plurality of pixels may be a partof a second tracker receiver, e.g. tracker receiver 236 of FIG. 2 . Thetwo tracker receivers may thus form an image sensor arrangement, whereina first image can be recorded by the first imaging region and a secondimage can be recorded by the second imaging region. Each image mayinclude a scene including an object which constitutes the target to betracked.

The control unit 730 is configured to receive a timing signal asillustrated in FIG. 7A, which indicates a time duration during which anillumination unit illuminating the target is switched on and off. Forexample, the presence of this signal may indicate when an illuminationunit, such as a tracker emitter, is switched on, and the absence of thissignal may indicate when an illumination unit is switched off. Byswitching on and off an illumination light of a scene including apreferably reflective target, it is possible to obtain two images onewith a bright, e.g. light reflecting, target and one with a dark target,i.e. no illumination light is emitted/reflected from the target.

In more detail, the tracker control unit 730 can control the firstimaging region 710 to take the first image of the scene when the timingsignal indicates that the illumination unit is switched on (or off), andcan similarly control the second imaging region 720 to take the secondimage when the timing signal indicates that the illumination unit isswitched off (or on). Hence, the pixels of the first imaging region 710record image information of the scene with illumination and the pixelsof the second imaging region 720 record image information of the scenewithout illumination.

When “first” and “second” are used in this description, it should beunderstood that these terms shall not be construed to give anylimitation to the specific time sequence. In other words, taking a“second image” may come in time before taking a “first image” so thatthe image which is taken first in time is taken while the illuminationunit is switched off and the other image is taken while the illuminationis on. Hence, the terms “first” and “second” are merely used todistinguish two different imaging regions and images which are taken attwo different times.

The read out unit 740 then reads out the first image from the firstimaging region 710 and the second image from the second imaging region720 and obtains a difference image. To obtain the difference image, theread out unit 740 determines a difference between the pixel values ofthe pixels of the first imaging region and the second imaging region.That is, by subtracting a pixel value of the first imaging region from apixel value of the second imaging region belonging to the same orsimilar part of the scene, the image information of the scenes with andwithout illumination are subtracted leaving a difference image, whichshows only the illuminated target and eliminates other highly reflectivestructures in the scene which could be erroneously considered areflective target by the tracker. Accordingly, the target may beidentified in the difference image with high accuracy.

The subtraction of a second image from a first image, or vice versa, isqualitatively illustrated in FIG. 7B. In this figure, the imaging region710 recorded a scene without illumination so that no target is seen andthe imaging region 720 recorded a scene with illumination so that target760 can be obtained as an absolute value independent of the sequence ofsubtraction. The difference image 750 is obtained by subtracting thepixels of the imaging region 720 from the pixels of the imaging region710, which correspond to the same position, i.e. the pixel values aresubtracted from each other which have recorded the same or similar partsof the scene. As becomes evident from FIG. 7B, the second image takenwith illumination on may also be taken before the first image or viceversa. Since the target is only seen in the image with illumination onand the rest of the scene (basically considered noise) is roughly thesame in both images, the result of the subtraction is a clear image ofthe target, independent of whether an image with illumination on andthen an image with illumination off is taken, or vice versa.

The pixels of the imaging regions 710, 720 may be pixels of acharge-coupled device (CCD) camera chip or a complementary metal-oxidesemiconductor (CMOS) camera chip. The sensor arrays of such camera chipsmay be composed of 1000×1000 sensor elements or more to generate digitalimages with 10⁶ image pixels (1 megapixel) or more. However, alsosmaller sensor arrays are feasible, for example, composed of 480×750sensor elements.

As known in the art of CCD or CMOS camera chips, pixel responses are notnecessarily the same in different chips and faulty pixels, e.g. hotpixels, may lead to always on or always off pixels. Thus, subtraction ofimage information of two different imaging regions of different chipsrecording the exact same scene does not necessarily lead to a completereduction of image information to zero. However, remaining pixel valuesmay be set to zero electronically in a calibration procedure.

Compared to taking two images with one imaging region and reading outthe imaging region before taking a second image, the time to take asecond image can be drastically shortened using two imaging regions.This particularly reduces the risk that the scene changes between twoexposures which could lead to the difference image including artifacts.For example, a car may drive through the scene or the scene may changeif the tracker is moved recording a different scene in the second imagecompared to the first image. Accordingly, the sensitivity in detectingand tracking a target is increased, since, for example, bright orreflecting surfaces which could be confused with a target can be largelyeliminated in the difference image. As a result, tracking can beperformed more reliably decreasing target losses when tracking a targetand decreasing the time in finding a target. For example, only theexposure time and time of flight of the tracking light may be betweentaking the images.

Moreover, even if the time between taking the first and the second imageis short, the read out unit 740 may correct for a possible movement ofthe tracker in the time between taking the first and the second image bytaking into account an offset between the scene on the first image andthe scene on the second image. The offset in this case corresponds to ashift in the scene due to the movement of the tracker. However, as canbe seen in FIG. 7B, if the target is kept roughly in the middle of theimage, it is possible to account for the offset by electronicallyshifting the scene. For example, if the scenes recorded in the imagingregions 710, 720 do not fully match (e.g. parts of the scene on the leftside are not present anymore in the second image due to movement of thetracker to the right) but only two thirds of the pixel columns on theright side of the first imaging region (see dashed rectangle) match twothirds of the pixel columns on the left side of the second imagingregion, it is sufficient to subtract only two thirds of the imagingregions to obtain the target. Since the angle positions at which thescenes were taken (with and without illumination), this subtraction ofonly parts of the imaging regions can be performed very accurately.

According to an embodiment, a method for tracking a target carried outby tracker 700 may comprise receiving in a first step a timing signalindicating a time duration during which an illumination of the target ina scene is switched on. In a second step, the method may comprise takinga first image of the scene including the target when the timing signalindicates that the illumination is switched on (or off) and then takinga second image when the timing signal indicates that the illuminationunit is switched off (or on). Finally, the method further comprisesreading out the first image from a first imaging region and the secondimage from a second imaging region and determining a difference betweenthe pixel values of the pixels of the first imaging region and thesecond imaging region so as to identify the target in a differenceimage.

The tracker 700 of FIG. 7A can be incorporated in different surveyingapparatuses. For example, in FIG. 2 the tracker is incorporated astracker 230, wherein the image sensor arrangement comprises the trackerreceiver 235 including an imaging region (see hatched area) and thetracker receiver 236 including another imaging region (see hatchedarea). Back-reflected tracking light is split by the beam splitter 255which is discussed in more detail below. The same optical arrangement isalso shown in FIGS. 3 and 4 where elements 335, 336 and 355 correspondto elements 235, 236 and 255.

Using two individual imaging regions in the tracker provides also forsimple ways of alignment. In a mechanical alignment, the first imagingregion may be glued on a beam splitting cube and the second imagingregion may be glued depending on the position of the first imagingregion. Additionally, electronic image transformation can be used foralignment or for a more precise alignment even before operation in thefield. In particular, the geometrical center of the two dimensionalarray of pixels of the first imaging region and the geometrical centerof the second imaging region do not need to coincide with each other.For example, in a calibration step, the tracker is calibrated by settingthe center of the second imaging region depending on the optical centerof the first imaging region. In more detail, a light spot in the centerof the first imaging region is also recorded in the second imagingregion and the position on which the spot is incident on the secondimaging region is defined as center. Further, a calibration method maybe provided in which the same target is recorded with the first imagingregion and the second imaging region so that using the obtainedcoordinates enable the extraction of parameters for a transformationbetween the two regions. For example, up to six parameters may be usedfor calibration, namely the six degrees of freedom. Additionally, byrecording the same image with two regions of two tracker receivers, thesensitivity of the different tracker receivers may be calibrated byapplying different gain factors.

The second tracker design is depicted in FIG. 8A and the setup islargely identical to the first tracker design depicted in FIG. 7A,merely the second imaging region 720 can be omitted. However, theimaging region 810, the control unit 830 and the read out unit 840 havea different functionality compared to the corresponding elements in FIG.7A; an example of the functionality will be discussed with respect toFIG. 8B.

The tracker 800 according to the second tracker design comprises animaging region 810, a control unit 830 and a read out unit 840.

The imaging region 810 having a plurality of pixels on an image sensorarrangement may be part of a tracker receiver, such as tracker receiver535 of FIG. 5 . In one step, the imaging region 810 takes a first imageof a scene including the target so that for each pixel of the imagingregion 810 charges being collected in a charge storage of the pixel andthe overall charges at all pixels correspond to the image information ofthe first image.

In another step, the imaging region 810 takes a second image of thescene including the target, wherein for each pixel a charge valuecorresponding to the image information of the second image is removedfrom the charge value of the collected charges corresponding to thefirst image. In essence, by removing charges, in the sense ofsubtracting or neutralizing charges, i.e. charge values constitutingvoltage potentials, of two images, a difference image can be generatedwithout first reading out the first image and second image.

The control unit 830 is configured to receive a timing signal asillustrated in FIG. 8A and discussed with respect to FIG. 7A, whereinthe timing signal indicates a time duration during which an illuminationunit illuminating the target in the scene is switched on and off. Byswitching on and off an illumination light of a scene including apreferably reflective target, it is possible to obtain a first and asecond image one with a bright, e.g. light reflecting, target and onewith a dark target, i.e. no illumination light is emitted/reflected fromthe target.

The control unit 830 controls the imaging region 810 to take the firstimage of the scene when the timing signal indicates that theillumination unit is switched on (off), and controls the imaging region810 to take the second image when the timing signal indicates that theillumination unit is switched off (on). Thus, the control basicallycorresponds to the control discussed with respect to control unit 730.

As mentioned above, “first” and “second” should not be construed to giveany limitation to a specific time sequence so that taking a “secondimage” may come in time before taking a “first image”. Hence, the terms“first” and “second” are merely used to distinguish two images which aretaken at two different times.

The read out unit 840 reads out, after taking the first and the secondimage and generating the difference image, the difference image from theimage sensor arrangement so as to identify the target in the differenceimage.

The charge storages of the pixels may have functions of a capacitor andregarded as part of the image sensor arrangement which may be part ofthe tracker receiver. For example, as mentioned above, the pixels of theimaging region 810 may be pixels of a charge-coupled device (CCD) camerachip or a complementary metal-oxide semiconductor (CMOS) camera chip.

These chips are usually made up of pixels, each of which can beconsidered to comprise a MOS (metal-oxide semiconductor) capacitor. Aslight, e.g. sunlight reflected from a scene and tracking light reflectedfrom a target, falls on each pixel, the incident photons hit thematerial of the pixel and knock electrons out of place according to thephotoelectric effect. These electrons are stored in the pixel'scapacitor. The charges may then build up as illustrated in FIG. 8B. Atthis stage, the image is still in analog form, with the charge, oramount of electrons on the capacitor, each pixel charge directlycorresponding to the amount of light that has hit it. Conventionally,the charges are read out for each image which may take severalmilliseconds.

In the tracker 800, the control unit is configured to reverse, aftertaking the first image, the polarity of pixels, so as to remove, i.e.subtract or neutralize/equalize, the charges corresponding to the imageinformation of the second image from the charges collected in the chargestorage corresponding to the image information of the first image. Inother words, the charge values of the charges collected in the chargestorage corresponding to the image information of the first image are atleast partly diminished (or even neutralized or equalized) by the chargevalues of the charges corresponding to the image information of thesecond image.

Reversing the polarity to achieve this “reduction” or“neutralization”-effect can be achieved in several ways, e.g. byswitching switches so that the charges of the first and second imagesare collected on opposite sides of a capacitor or by taking two imagesduring the pixel read-out cycle similar to correlated double sampling.

As illustrated in FIG. 8B, switches S1, S2, S3 and S4 are switchedbetween taking a first and taking a second image, which may be realizedby transistors. By setting switches S1 and S4 in the ON position (asshown in FIG. 8B), it is possible to basically store the electronsobtained from incident photons of the first image on one side of thecapacitor, which is an example of a charge storage, and the electronsobtained from the photons of the second image on the other side of thecapacitor. In more detail, to achieve this distribution, the controlunit 830 switches the switches after the first exposure such that forthe second image the lower side of the capacitor receives the electrons(switches S1 and S4 are switched from ON to OFF and switches S2 and S3from OFF to ON).

In this way, the pixel storage functionality allows to build thedifference of integrated signal values from two consecutive exposuressuch that the read signal equals to the difference of the two exposures.For example, if a pixel charge of the first and the second image isidentical, the same number of electrons is on opposite sides of thecapacitor so that no resulting voltage, hence signal, is obtained.Carrying out this process for all pixels of the imaging region leads toa difference image.

In correlated double sampling two images are taken, namely one when thepixel is still in the reset state and one when the charge has alreadybeen transferred to the read-out node. The two values of each pixel arethen used as differential signals in further stages, such asprogrammable gain amplifiers (PGA) or analog-digital-converter (ADC), sothat a difference image can be obtained. In essence, in the presentcase, the output of a sensor, such as a camera chip, is measured twice,once in an illumination “on” condition and once in an illumination “off”condition, and a difference of the measured values is taken, effectivelyreversing the polarity of the pixels. For example, correlated doublesampling can be used in switched capacitor operational amplifiers andwhen used in imagers, correlated double sampling is a noise reductiontechnique in which the reference voltage of the pixel, i.e., the pixel'svoltage after it is reset, is removed from the signal voltage of thepixel, i.e., the pixel's voltage at the end of integration, at the endof each integration period.

According to an embodiment, a method for tracking a target carried outby tracker 800 may comprise receiving in a first step a timing signalindicating a time duration during which an illumination of the target ina scene is switched on. In a second step, the method may comprise takinga first image of the scene including the target when the timing signalindicates that the illumination is switched on (or off) and then takinga second image when the timing signal indicates that the illuminationunit is switched off (or on). When taking the first image charges arecollected for each pixel in a charge storage corresponding to the imageinformation of the first image. When taking the second image, a chargevalue for each pixel corresponding to the image information of thesecond image is removed from the charge value of the collected chargescorresponding to the first image so as to generate a difference image.Finally, the method further comprises reading out the difference imageso as to identify the target in the difference image.

For both tracker designs it does not matter whether the tracker emitteris placed next to the tracker receiver (see e.g. FIG. 5 ) or placed atone end of the lens arrangement 110 (see e.g. FIG. 2 ), for example, andmay constitute a ring of LEDs around an opening of the lens arrangement.Indeed, instead of a tracker emitter, such as emitters 537 or 237,emitting light to be reflected at the target, the target itself mayinclude an illumination unit so that the target may be an active targetreceiving a timing signal to turn on or off the illumination unit. Thetwo tracker designs have different advantages and disadvantageous. Forexample, while the second tracker design has the disadvantage ofrequiring a more complex and expensive tracker receiver, it has theadvantage of avoiding the need for a transformation and for the use of a50/50 beam splitter so that less light is lost and approximately 100% ofthe light is incident on the imaging region 810. The two trackers 700and 800 constitute alternative solutions in providing a difference imagewithout having to wait until two images are read out from one or twotracker receivers. Both trackers can be used interchangeably in theapparatuses shown in FIGS. 1B, 1C, 2 and 5 , whereas FIGS. 2 and 5 arediscussed in more detail in the following.

FIG. 2 illustrates a detailed embodiment of a surveying apparatus 200.The surveying apparatus 200 is a detailed example of surveying apparatus100A, 100B, 100C. Hence, the elements, and particularly details thereof,discussed with respect to FIG. 2 can be combined with elements of thesurveying apparatuses 100A, 100B and 100C and vice versa.

In FIG. 2 , details of the lens arrangement, tracker, imaging unit anddistance measuring unit shown in the previous figures are discussed byreferring to lens arrangement 210, tracker 230, imaging unit 220 anddistance measuring unit 240.

The lens arrangement 210 is depicted with a housing and a front opening218. The opening 218 may comprise a fixed final focus lens on theoptical axis of the lens arrangement. Further, the lens arrangement 210comprises the focus lens element 116, which may be a compound lens or anindividual lens having preferably a convex side facing the beamsplitter/combiner 250. The focus lens element is arranged to be movablein the direction of the optical axis so as to provide different focussettings to sight an object.

The zoom lens element 214 may also be a compound lens or an individuallens and is arranged to be movable in the direction of the optical axis.The zoom lens element 214 is adapted to zoom. In other words, changingthe position of the zoom lens element, leads to a change in the field ofview. The focus lens element 116 and the zoom lens element 214 form azoom optic and can be moved by drives. In particular, the lensarrangement is configured to maintain a magnification ratio so that animage size of the object on the imaging unit 220 is maintained constant.This may be achieved by driving the movable focus lens element 116 andthe zoom lens element 214 accordingly.

Fixing the zoom optic to a constant magnification ratio has theadvantage that objects, such as reflectors, have the same sizesimplifying image processing. For example, an object of 1 cm in thefield has the same number of pixels in an image independent of whetherit is at a distance of 10 m or 40 m.

Similarly, for outgoing light, e.g. distance measuring light of thedistance measuring unit 240, the spot size of the outgoing laser lighthas always the same size on the object. Further, moving the lenselements 214, 116 depending on each other reduces the calibrationeffort.

The lens arrangement 210 is further configured to switch between anarrow field of view and a wide field of view. For example, by adjustingthe position of the zoom lens element 214 the field of view obtainableby the lens arrangement can be changed. A wide field of view may be usedto obtain an overview image of a scenery in which the object can beeasily found and measured and/or tracked, while a narrow field of viewmay be used for taking a distance measurement. For example, the widefield of view can be used to obtain panoramic images in whichmeasurement points can then be defined. Another advantage of panoramicimages is that the images can be used to remotely control the surveyingapparatus by viewing the images at a position remote to the surveyingapparatus.

The beam splitter/combiner 250 in FIG. 2 is the same one as describedwith respect to FIGS. 1A, 1B and 1C and may comprise two prisms gluedtogether. In another example, there may be an air gap between the twoprisms. In addition to the previous embodiments, interfaces to thefunctional modules 220 and 230 are shown in more detail. For example,the additional prism element 257 which may form part of the prism systemis provided to achieve a good optical connection to the imaging unit220. In this example, the imaging unit 220 is an electronic imagingdevice having a camera chip, such as a CCD, which can be glued to theprism element 257. This reduces the need of additional mechanicalcomponents for positioning, adjusting and/or fixing the electronicimaging device to the additional prism element. The additional prismelement 257 may also be glued to the beam splitter/combiner 250 so as toform a compact unit which should not require any optical adjustmentseven if mechanical shocks are applied.

The tracker channel of the multi-channel prism system 250 is indicatedby a dotted line showing an approximation of the optical axis of thetracker 230. The tracker 230 comprises two tracker receivers 235 and 236which may be realized by two camera chips schematically illustrated ashashed rectangles. One example of a tracker emitter is illustrated withreference sign 237. This tracker emitter may be made up of LEDs arrangedin a ring surrounding the front opening 218 forming an LED array, whichis located away from the tracker 230. These light-emitting diodes (LEDs)may emit infrared light in the same direction as the optical axis of thelens arrangement 210. Tracking light reflected from the object is thenreceived in the lens arrangement 210 and split by the beamsplitter/combiner 250 to follow the optical tracker path before beingincident on beam splitter 255 which may be a single transparent mirroror prism cube. The beam splitter 255 divides the incoming light into twoparts of back-reflected tracking light each preferably comprising 50% ofthe received intensity.

Accordingly, two images of tracking light reflected by an object andreceived by the surveying apparatus are obtained either in parallel orsequentially depending on when an image should be acquired. In the sameway as discussed with respect to the additional prism element 257, thebeam splitter 255 can be glued to the beam splitter/combiner 250 and tothe camera chips of the tracker receivers.

In one embodiment, the first image may be acquired when the trackeremitter 237 is on and illuminates the object and the second image may beacquired shortly after when the tracker emitter 237 is off. In the sameway, the first image may be acquired when the tracker emitter 237 is offand the second image may be acquired shortly after when the trackeremitter 237 is on and illuminates the object. As discussed in detailabove with respect to FIGS. 7A and 7B, by subtracting the images adifference image of the tracking light reflected at the object can bederived.

The tracker 230 of FIG. 2 has the tracker design of tracker 700.However, alternatively, also the tracker design of tracker 800 may beapplied in FIG. 2 .

The distance measuring unit 240 in FIG. 2 shows a detailed example ofthe distance measuring unit 140 comprising a laser 244 and a detector246 in the same module and having the same optical path for the laseremitter and detector. The laser may emit light in the red, as discussedwith respect to FIG. 1A, or in the infrared wavelength range. The laserof the distance measuring unit 240 is adapted to emit laser light whichis reflected by the beam splitter/combiner 250 so as to be outputcoaxially to the optical axis of the lens arrangement.

As schematically illustrated in FIG. 2 , the laser light may follow thedashed line first passing the apertured mirror 248 (a.k.a. pinholemirror) and then entering the beam splitter/combiner 250 where it istwice reflected before being outputted to the lens arrangement 210.After passing the two lenses elements 116 and 214 the focused laserlight exits at the front opening 218 and is then reflected by an object(not shown).

The reflected laser light again passes through the lens arrangement 210,is reflected twice in the beam splitter/combiner 250 and is incident onthe apertured mirror 248 in the distance measuring unit 240.Alternatively, a beam splitter instead of the aperture mirror (a highlyreflective mirror with a pinhole to allow the laser light going through)can be used. This beam splitter may be a 50:50 beam splitter and partsof the reflected laser light are then detected on the detector 246. Thedetector 246 may be an avalanche photodiode.

Once the detector detects back-reflected measuring light, a controllerof the surveying apparatus may use known methods, such as a pulse methodor phase method, to perform a distance measurement.

FIG. 3 illustrates a part of the surveying apparatus 100A, 100B, 100C,200 having an alternative arrangement of functional modules.

The beam splitter/combiner 350 again comprises two wedge shaped prismswhich, however, are arranged differently to beam splitter/combiner 250so that also the optical paths are different. In FIG. 3 theimaging/visual channel is located on the top and the visible lightcoming from the lens arrangement needs to be reflected twice to reachthe imaging unit 320. The tracker 330 is again constituted by twotracker receivers 335 and 336 which both receive reflected trackinglight from a beam splitter 355. The distance measuring unit 240 is thesame as in FIG. 2 .

FIG. 4 illustrates a specific example of a tracker which can be usedtogether with the surveying apparatus.

The tracker 330′ illustrated in FIG. 4 comprises two tracker receivers335, 336 and the tracker emitter 337 in the same functional module 330′.It can be understood that the tracker 330′ can replace the tracker 330in FIG. 3 or the tracker 230 in FIG. 2 . The advantage of the tracker330′ over the other trackers is that the light of the tracker emitter337 has largely the same optical tracker path as the light falling onthe tracker receivers. Further, since the tracking light of the trackeremitter 337 passes through the lens arrangement and its lenses, thelight can be focused on the object so as to receive a strongerreflection back compared to the case of the tracker emitter 237.However, providing two beam splitters, as shown in FIG. 4 , to arrangethe optical axes of both tracker receivers and the tracker emitter onthe same optical axis as the lens arrangement introduces more opticalcomponents and thus more complexity.

FIG. 5 illustrates a specific embodiment of a surveying apparatus 500including details of the lens arrangement, tracker, imaging unit anddistance measuring unit shown in the previous figures and referred to aslens arrangement 510, tracker 530, imaging unit 520 and distancemeasuring unit 540 in the following.

Specifically FIG. 5 shows an optical path 580 of the tracker (opticaltracker path) by the dotted line passing the edges of the lenses in thelens arrangement 510 including the final focus lens 518, the zoom lenselement 514 represented as compound lens, and the focus lens element 516represented as compound lens, wherein the focus lens element 516 ismovably arranged between the final focus lens and the beamsplitter/combiner.

The optical tracker path 580 is further indicated in the prism system550 having multiple reflections at the surfaces and further indicated inthe beam splitter cube 555 to which the tracker emitter 537 and thetracker receiver 535 is attached in this example. The optical trackerpath 580 can be filled with light from the tracker emitter 537, e.g.comprising one or more infrared (IR) LEDs or an infrared laser atapproximately 850 nm. When the tracking light exits the lens arrangement510 and hits the object 505, which is preferably a reflector made of atriple prism in this example, the reflected light enters again the lensarrangement 510 as a light beam reflected from the target. This lightbeam is indicated by the dashed line 590. Reflections of this dashedline in the prism system and the position where the light beam hits thetracker receiver 535, e.g. an IR camera chip, is also illustrated inFIG. 5 .

In FIG. 5 , the tracker 530 may have a tracker with the functionality oftracker 800. However, alternatively, also the tracker design of tracker700 may be applied in FIG. 5 . For example, tracker 530 may be replacedby tracker 330′ of FIG. 4 . In another example, tracker 530 may bereplaced by tracker 230 or 330 as illustrated in FIGS. 2 and 3 ,respectively. In this case, the tracker emitter is preferably adapted asdiscussed with respect to tracker emitter 237, e.g. as an LED ring. Forboth tracker designs, it does not matter whether the tracker emitter isplaced next to the tracker receiver (see e.g. FIG. 5 ) or placed at oneend of the lens arrangement 110 (see e.g. FIG. 2 ).

Additionally, the reflector 505 and its surroundings may be imaged bythe imaging unit 520 which receives visible light through the imagingchannel comprising the optical element 557.

The distance measuring unit 540 is discussed in the following. Similarto the distance measuring units 140 and 240, the distance measuring unit540 comprises a laser 544 and a detector 546. The distance measuringunit 540 is adapted to emit laser light from the laser 544 which passesthrough the beam splitter 548 and the lens 549 before being incident onthe beam splitter/combiner 550. The laser wavelength may be in the redwavelength range, such as 635 nm, and the beam splitter/combiner 550 isadapted to reflect the laser light so as to overlap with the opticalaxis of the lens arrangement 510 when exiting the beam splitter/combiner550, illustrated as prism system in FIG. 5 .

Therefore, the laser light must pass several optical elements 548, 549,550, 516, 514, 518 before being reflected by the reflector 505. Hence,back-reflected light may be detected not only from the reflector 505 butalso from the other optical elements which could lead to the wrongassumptions regarding the distance to the reflector 505.

In detail, the time of flight of a laser pulse from the laser 544 to thereflector 505 may be used to measure the distance and if one of theseveral optical elements of the surveying apparatus also provides areflection which can be detected by the detector 546, the distancemeasurement result may not be reliable. Therefore, care has been takento avoid any undesired (multipath) reflections. For example, lenselements 549, 516, 514 and 518 are provided with anti-reflectioncoatings. Further, the lens element 549 may be chosen such that itfocuses the laser light emitted by the laser 544 on an intermediatefocus between the lens element 549 and the lower surface of the beamsplitter/receiver 550 on which the laser light is incident. Inparticular, it is desired to avoid an intermediate focus on the prismsurface facing the distance measuring unit 540 which could lead tostrong back-reflections. Furthermore, this prism surface may be providedwith an anti-reflection coating as well.

Additionally, the right lens of the focus lens element 516, i.e. thelens facing the beam splitter/combiner 550, has a convex side facing thebeam splitter/combiner 550. This lens may be a plano-convex or abi-convex lens, as shown. As a result, reflections from this lens ofdistance measuring light from the distance measuring unit 540 may noteasily reflect back into the distance measuring unit 540 and reach thedetector 546. Therefore, crosstalk which could lead to the detections ofsignals not coming from the actual target can be largely avoided.

Crosstalk can further be suppressed by the optical design andorientation of the prism system 550 and the lens groups 514 and 516. Forexample, the surfaces of the prism system 550 on which light is incidentcan be slightly tilted with respect to an orthogonal direction so thatlight is not fully orthogonally incident thereon. Furthermore, air gapsbetween individual prisms for total reflection can be provided whereappropriate.

According to the above, there are several challenges when guiding thelaser light of the distance measuring unit 540 through the prism system550 and the lens arrangement 510 which have hindered the skilled personto consider such a coaxial alignment. By taking care of undesired backreflections, as outlined in the examples above, and by intelligentlyprocessing back-reflected light detected by the detector 546, the riskof unreliable measurement results can be mitigated.

For intelligent processing detected light, the surveying apparatus maycomprise a controller. The controller, such as controller 608 of FIG. 6, may apply an electronic gating method, e.g. ignoring detected backreflections which are received very shortly after the laser pulse hasbeen emitted so that only reasonable distances to an object of more than2 m are considered, for example. In particular, the controller may beprogrammed to ignore detected back reflections which lead to a distancebelow a predetermined threshold value, e.g. 2 m. The functions ofcontroller 608 may be distributed over the individual units of thesurveying apparatus 600. In particular, the control function of theelectronic gating method may be provided directly in the distancemeasuring unit 640.

On the other hand, using the optical setup shown in FIG. 5 , theperformance of the distance measuring unit, the tracker and lensarrangement can be improved compared to simple bi-axial systems, inwhich the optical axis of the lens arrangement and the optical axis ofthe laser measuring unit do not overlap. In the surveying apparatus 500of FIG. 5 , when guiding the laser light of the distance measuring unit540 through the prism system 550 and the lens arrangement 510, theoptics of the lens arrangement can be controlled to focus the laserlight on the reflector 505 which leads to a better signal-to-noise ratiowhen receiving back reflected light. Additionally, the focused laserspot on the reflecting object is smaller than without focusing so that ahigher resolution of the object can be obtained.

FIG. 6 illustrates elements of a surveying apparatus 600 in a surveyingsystem according to another embodiment emphasizing the communication andcontrol between elements on a functional level. In addition to thesurveying apparatus 600, the system may comprise the remote control unit690 which can be used by an operator to control the surveying apparatus690 or total station including such an apparatus.

The surveying apparatus 600 comprises an acquisition unit 615, acontroller 608, memory 605 and remote communication unit 612 which maycommunicate with the remote control unit 690 to receive instructionstherefrom.

The acquisition unit 615 may form a part of the head of a total stationand comprises the lens arrangement 610 including the lens elements 214and 116, the imaging unit 620, the tracker 630 and the distancemeasuring unit 640. Since the illustration in FIG. 6 is not concernedwith the optical setup but explains the surveying apparatus 600 on afunctional level, details about a prism system and the combination ofdifferent optical paths have been avoided and it is referred to theprevious figures for details. The acquisition unit 615 further comprisesa positioning unit 625 which is provided for adjusting the optical axisof the lens arrangement 610 relative to a reference axis, such as anaxis of a polar coordinate system including a distance and two angles.For example, the positioning unit is realized by an electromechanicalarrangement comprising preferably servo drives for precisely positioningthe acquisition unit 615.

Accordingly, the positioning unit 625 may move the lens arrangement 610to follow a moving object. In detail, when the optical arrangement 610sights an object, the tracker 620 may track the sighted object. Thetracker 620 may evaluate the movement of the object, e.g. in thecontroller 608, and may issue an instruction to the positioning unit 625to move the optical axis of the lens arrangement. In this way, the headof the surveying apparatus or total station including the surveyingapparatus may be moved to follow the object (target).

The control of the functional modules may constitute individual controlelements controlling each module individually and being located close toor in the functional modules. The control elements may be realized by ahardware arrangement, such as hard-wired circuits or ASICs (applicationspecific integrated circuits) or software or any suitable combination ofthe above. In particular, the control of the functions performed by thelens arrangement 610, the tracker 630, the imaging unit 620 and thedistance measuring unit 640 may be realized by software.

In the surveying apparatus 600 of FIG. 6 individual control elements arecombined in the controller 608. For example, a tracking control element,an imaging control element and a distance measuring control element maybe realized by a processor running different software codes which may bestored in the memory 605.

Furthermore, the controller 608 may be configured to analyze the imageof the object acquired by the imaging unit 620 and configured to issuean instruction to the lens arrangement 610 to move one or more lenses ofthe lens arrangement so as to maintain an image size of the object onthe imaging unit constant and/or lens movement may be used for theautofocus. In the same way, the controller 608 may be configured toanalyze the image of the object acquired by the tracker receiver of thetracker 630 and configured to issue an instruction to the lensarrangement 610 to move one or more lenses of the lens arrangement so asto maintain an image size of the object on the tracker receiverconstant. Further, the controller 608 may be configured to analyze theback-reflected light detected by the detector of the distance measuringunit and calculate the distance to the object by considering the timewhen the laser pulse of the laser was emitted and the time when theback-reflected light was detected.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the trackers and apparatusesof the invention without departing from the scope of or spirit of theinvention.

The invention has been described in relation to particular exampleswhich are intended in all respect to be illustrative rather thanrestrictive. Those skilled in the art will appreciate that manydifferent combinations of hardware, software and firmware will besuitable for practicing the present invention.

Moreover, other implementations of the invention will be apparent tothose skilled in the art from consideration of the specification andpractice of the invention disclosed herein. It is intended that thespecification and the examples be considered as exemplary only. To thisend, it is to be understood that inventive aspect lie in less than allfeatures of a single foregoing disclosed implementation orconfiguration. Thus, the true scope and spirit of the invention isindicated by the following claims.

The invention claimed is:
 1. Tracker of a surveying apparatus fortracking a target, comprising: an imaging region having a plurality ofpixels on an image sensor arrangement for taking: a first image of ascene including the target, for each pixel charges being collected in acharge storage corresponding to image information of the first image,and a second image of the scene including the target, for each pixel acharge value corresponding to the image information of the second imagebeing removed from the charge value of the collected chargescorresponding to the first image so as to generate a difference image; acontrol unit configured to: receive a timing signal, the timing signalindicating a time duration during which an illumination unitilluminating the target in the scene is switched on and off, control theimaging region to take the first image of the scene when the timingsignal indicates that the illumination unit is switched on, and controlthe imaging region to take the second image when the timing signalindicates that the illumination unit is switched off; and a read outunit configured, after taking the first and the second image, to correctfor movement of the tracker in a time between taking the first image andthe second image by taking into account an offset between the scene onthe first image and the scene on the second image, the offsetcorresponding to the movement of the tracker, and to read out thedifference image from the image sensor arrangement so as to identify thetarget in the difference image.
 2. The tracker according to claim 1,wherein the control unit is configured to reverse, after taking thefirst image, polarity of the pixels, so as to remove the chargescorresponding to the image information of the second image from thecharges collected in the charge storage corresponding to the imageinformation of the first image.
 3. The tracker according to claim 1,wherein the tracker comprises a tracker emitter for emitting trackinglight on an optical tracker path.
 4. The tracker according to claim 1,wherein the tracker is adapted to issue an instruction to a surveyingapparatus to move an optical axis of a lens arrangement of the surveyingapparatus.
 5. The tracker according to claim 1, wherein the chargestorage includes a capacitor.
 6. The tracker according to claim 2,wherein the tracker comprises a tracker emitter for emitting trackinglight on an optical tracker path.
 7. The tracker according to claim 2,wherein the tracker is adapted to issue an instruction to a surveyingapparatus to move an optical axis of a lens arrangement of the surveyingapparatus.
 8. The tracker according to claim 2, wherein the chargestorage includes a capacitor.
 9. Method for tracking a target using atracker of a surveying apparatus, comprising: taking a first image of ascene including the target on an imaging region having a plurality ofpixels, for each pixel charges being collected in a charge storagecorresponding to image information of the first image; and taking asecond image of the scene including the target on the imaging region,for each pixel a charge value corresponding to the image information ofthe second image being removed from the charge value of the collectedcharges corresponding to the first image so as to generate a differenceimage; wherein a timing signal is received, the timing signal indicatinga time duration during which an illumination of the target in the sceneis switched on and off, and the imaging region is controlled to take thefirst image of the scene when the timing signal indicates that anillumination unit is switched on, and the imaging region is controlledto take the second image when the timing signal indicates that theillumination unit is switched off; and the method further comprises:correcting for a movement of the tracker in a time between taking thefirst image and the second image by taking into account an offsetbetween the scene on the first image and the scene on the second image,the offset corresponding to the movement of the tracker, and reading outthe difference image from the imaging region so as to identify thetarget in the difference image.
 10. A computer system including aprocessor configured to execute a program including instructions adaptedto cause data processing means to carry out the method of claim
 9. 11. Anon-transitory computer readable medium in which the program of claim 10is embodied.